1. Field of the Invention
This invention pertains to the field of gearshift controls for automatic transmissions. More particularly, it pertains to a system for controlling operation of solenoid-operated valves that supply pressurized hydraulic fluid to clutches in the transmission. Engagement and release of the clutches cause the gearset to change the operating gear ratio. A feedforward control uses a net engine torque signal and the inverse frequency response to commanded engine torque of the solenoid valve and the associated transmission clutch.
2. Description of the Prior Art
U.S. Pat. No. 4,757,886 describes a closed loop control system for controlling operation of a transmission bypass clutch Particularly on the basis of slip across the torque converter. The system employs measured slip as a feedback signal combined with a command signal to produce a slip error. Thereafter, a duty cycle correction formed on the basis of the difference between duty cycles in the current sampling period and the previous sampling period is developed and processed to produce a clutch pressure signal used to vary slip and as feedback to enhance stability of the duty cycle error signal.
U.S. Pat. No. 4,790,418 describes a technique for controlling the transfer of torque from an offgoing friction element to an oncoming friction element when a gear ratio change is being made in an automatic transmission. Slip across the offgoing clutch is used to control engine speed. Time rate of change of torque in the offgoing clutch is used as a basis to change clutch pressure in accordance with a calculated torque slope.
U.S. Pat. No. 4,792,902 describes a system and a method for controlling the ignition timing of an internal combustion engine in a driveline that includes a multiple speed ratio transmission. Spark timing of the engine is advanced and retarded on the basis of engine speed and transmission output speed by a control algorithm whose execution improves the gear ratio changes. None of these patents describes a feedforward technique for controlling the friction element of an automatic transmission.
When a gear ratio change occurs in an automatic transmission, the variation of the speed ratio output by the transmission with time is characterized by two phases, a torque phase and an inertia phase. During the torque phase, torque produced by the engine is exchanged between an offgoing friction element and an oncoming friction element so that, when these elements are fully engaged and fully released, the speed ratio of the transmission associated with the gear ratio at the end of the gearshift results. During the inertia phase of an upshift, the oncoming friction element controls the speed of the transmission input shaft to a new lower value corresponding to the gear ratio after the shift is completed. This effectively controls the speed ratio produced by the transmission because, over the short duration of a gear shift, the speed of the transmission output shaft is constant.
The strongest disturbance influencing this transfer of torque occurs when a change of engine torque is demanded by the vehicle operator when changing the position of the throttle valve by depressing or releasing the accelerator pedal. The effects of these disturbances are particularly acute and significant when the driver's manual control of the accelerator pedal is inconsistent with previous action that initiated the gear ratio change. For example, if during the inertia phase, the driver suddenly depresses the accelerator pedal, engine torque increases, thereby causing more slip across the oncoming friction element. This action causes reduction in fuel economy, losses associated with inefficient operation of the torque converter, and excessive wear of the friction element. If during the inertia phase, the accelerator pedal is suddenly released, potential for friction element to lock or to slip only slightly, leads to unacceptable noise, vibration and harshness.